Rubber derivative and method of making same



Patented Feb. 23, 1943 UNITED STATES PATENT OFFICE RUBBER DERIVATIVE ANDlvIETI-IGD OF MAKING SAME No Drawing. Application June 2, 1939, SerialNo. 277,086

12 Claims.

This application is a continuation in part of my co-pending applicationSerial No. 214,666, filed June 20, 1938, and the invention relates torubber derivatives and a process for making the same, and, inparticular, to rubber derivatives suitable for bonding rubber to metaland other surfaces, and as a basis for coating compositions, as paints.varnishes, and coatings. It also relates to a process for bonding rubberto metal, fabric, solids, and the like.

Several methods have been developed for bonding rubber to metal, butbecause of the complicated procedure involved, or because of thecharacteristics and strength of the bond produced, these methods arefrequently unsatisfactory. Some methods require the adhesive cementcoating for bonding rubber to metal to be heated under accuratelycontrolled conditions to modify it before the rubber is vulcanizedthereon. In other methods a thermoplastic adhesive is used, and themoldmust be cooled before the rubber and metal assembly is removed.

It is, therefore, an object of this invention to provide a simplifiedmethod for firmly uniting rubber to metal and other materials, whereinthe assembly may be removed hot from the mold without destroying thestrength of the union.

It is another object of this invention to provide rubber derivatives,and a process for making the same, which derivatives may be used to forma bond of improved strength between rubber and metal or the like.

It is a further object of this invention to provide an adhesive, and amethod for producing the same, which possesses excellent aging qualitiesand relatively high bonding strength at elevated temperatures.

It is a still further object of this invention to provide a rubberderivative having superior aging properties and substantial solubilityin rubber solvents, and which, when suitably compounded, may be used asa protective coating having relatively high resistance to atmosphericconditions.

The improved rubber derivatives of this invention are prepared bymilling sulfur or equivalent rubber vulcanizing agents, such asselenium, vulcanizing types of accelerators, etc., and one or moresuitable conversion reagents, into rubber, such as Hevea smoked sheet,or pale crepe, sheeting the product out to relatively thin sheets,heating the sheets under suitable conditions until the desired reactiontakes place, and then masticating the product to render it soluble inrubber solvents. To obtain the improved rubber derivative of thisinvention the sulfur must be chemically combined in the resulting rubberderivative in rather definite proportions. Suitable rubber acceleratorsmay be added to the mix, if desired.

The properties of the rubber derivative produced depend to a largeextent on the conversion reagent used in its formation, certain reagentsgiving adhesives having considerably improved properties. Conversionreagents which give products with good properties have been found to be,in general, mixtures of one or more salts of a strong acid with a weaklyacidic substance. These compounds are considered to have an oxidizingaction on rubber when mixed with Weak acids, that is, they reduce thenumber of double bonds present in the derivative. The character of therubber derivative produced seems to be improved when both of theingredients are solids and may be further improved when one or more ofthe solid substances contains considerable water of crystallization. Thesulfates, and, in particular, the acid sulfates, are preferred,especially when they contain considerable water of hydration. For bestresults, these materials are used with a suitable cooperatingingredient, such as a weakly acidic substance. Aluminum acid sulfate,when properly used with a cooperating material of the character ofphosphoric acid, produces rubber derivatives having exceptionallydesirable properties for use both as an adhesive and as a basis forpaints.

Examples of the acid sulfates which are particularly desirable for useare acid sulfates of aluminum, iron (both ferric and ferrous), alkalimetals, including ammonia, alkaline earth metals, the closely relatedalums, potassium alum and other alums. In addition to the acid sulfates,certain sulfates may be used advantageously. Examples of these aresulfates of iron (both ferric and ferrous), copper, mercury, andcadmium. Examples of salts of strong acids other than sulphuric acid arebromides of mercury and zinc, chlorides of mercury, tin and aluminum,and. alkali metal iodides. Phosphoric acid is a very good example of aweakly acidic substance desirable for use in forming the rubberconversion product hereinbefore mentioned. For some uses, where it isdesirable to have the conversion product somewhat more soluble in rubbersolvents, oxalic acid has been used advantageously. Other Weak acidsthat are also desirable for use are benzoic, tartaric, phthalic andsalicylic. The particular acid may be selected for the properties itgives to the conversion product, as various acids seem to impartslightly different properties.

These weakly acidic materials, while assisting in the formation of therubber derivatives, are not absolutely essential ingredients of theconversion reagents, and may, in some cases, be omitted. The quantity ofsalts of strong acids required to convert the rubber into the derivativeis, however, much larger when the weakly acidic compound is omitted as apart of the conversion reagent. Thus, the rubber derivative may beprepared by using aluminum acid sulfate alone as the conversion reagent,but its properties as an adhesive are improved and the quantity ofreagents required is decreased when phosphoric acid is also present.

It is to be noted that the phosphoric anhydride, P205, may be used inplace of the phosphoric acid if some water is present in the conversionreagent mixture. In the cases of the other weak acids, if the anhydrideis readily available it may be used, provided a small amount of water ispresent in the mixture.

In-the preparation of the rubber derivatives, the conversion reagents,sulfur, and, if desirable, an accelerator, are mixed or milled into therubher until the ingredients are thoroughly incorporated. Ordinarilythere should be present in the mix,'before heating, from about 3% to ofwater, which includes water of crystallization'and/or free water whichis preferably added during the mixing. The amount of total water mayvary outside of and beyond these limits for the production of materialswith desired specific properties. The so mixed product is preferablyformed into relatively thin section, such'as relatively thin sheets, ofan appropriate thickness of about 2 or 3 mm. orso, which are placed inan oven with temperature control and heated for an appropriate period.During the heating, an exothermic reaction takes place, and, to obtainthe most desirable properties, it is essential that the temperature inthe sheets does not rise substantially higher than 170 C., and it isdesirable to keep the temperature of the materialas low as possible andyet obtain reaction within a reasonable time. This is obtained byutilizing sheets of 2 or 3 mm. in thickness. Also, evaporation of thewater contained in the sheetse'rves to keep the temperature down. Ifdesired, an-inert gas or air may be circulated over the sheeted materialin the oven. It is preferred, however, to heat the material in a relatively high vacuum, such as about 500 mm. or more, of mercury. Bycarrying out the reaction in a vacuum or an inert gas, the tendency forportions of the thin sheets to become harder than others issubstantially eliminated, and a 1 more uniform product is obtained.

By the use of very thin sheets, such as 1 mm. or so, .in thickness,,itis possible to carry on the reaction "at temperatures as low as 100 C.to 105 C., anda derivative having higher molecular weight is produced,which is relatively less soluble. The formation of the material in thinsection is important in facilitating the reaction, it being found thatthe reaction takes place with difiiculty when relatively thick sheetsare used.

The sheeted material is heated for a sufficient time to allow thereaction to'progress to the desired state. A somewhat harder product isproduced by heating a given mixture for a longer period of time. Afterheating, the material is substantially insoluble in rubber solvents, butsolubility may be restored by masticating the material for a sufiicienttime on a rubber mill or in a suitable mixer, if the molecular weight ofthe product is not too high.

The quantity of conversion reagent used in forming the rubber derivativeis dependent on the character of the derivative desired and theconversion reagent used. Larger quantities of conversion reagent, suchas aluminum acid sulfate with phosphoric acid, give harder products whenheated for a given length of time. When a weakly acidic compound is notpresent, a larger quantity of the acid salt of strong acid must be used,and this may amount to 50% or more of the rubber mixture. When a weakacid, such as phosphoric, is also present, the total of both ingredientsof the conversion reagent may, in some cases, be less than 10% of therubber mixture.

The sulfur greatly improves the strength and aging properties of theadhesive material at ele- 'vated temperatures, and improves the colorand resistance of paint films containing the derivative. The improvementin the properties of the derivative is very noticeable, even when smallpercentages, such as .l%, are used, and, when more than about 3% or soof sulfur is present, the derivatives tend to be insoluble, and thesolubility is not restored by milling or mastication. Hence, rubberderivatives having more than about 3% combined sulfur are unsuitable formany uses. The effect of sulfur on the solubility of the rubberderivatives is greater with the softer material than with the hardermaterials; thus, a smaller quantity of sulfur than 3% may render a softderivative insoluble. It is preferred to use the maximum amount ofsulfur in the product commensurate with the solubility desired.

The improved rubber derivatives, having the same properties as above,may also be prepared by adding sulfur, or its equivalent, to thehydrocarbon conversion products obtained by mixing or milling aconversion reagent, such as those mentioned, into the rubber, sheeting,and heating, as above described, but without sulfur being present. Whenthe sulfur is added to this hydrocarbon conversion product it isthoroughly mixed therewith, and, if desired, an accelerator is added andthe mixture heated to vulcanizing temperature, to insure combination ofthe sulfur with the hydrocarbon obtained from the rubber.

The sulfur may be added as elemental sulfur, in the usual manner, to therubber mix, or to the conversion product, or it may be added in the formof a vulcanizing type of accelerator, such astetramethylthiuramdisulfide, etc. If the amount of combined sulfur isgreater than is commensurate with the required solubility, it has beenfound that the required solubility may usually be restored by millingthe sulfur-containing rubber derivative with sufficient of a moresoluble conversion product, which results in a decrease in thepercentage of sulfur present and lowering of the viscosity of resultingsolutions. Thus, for example, the hard, insoluble rubber derivativecontaining 4% of sulfur may be solubilized by milling it either with anequal quantity of a derivative having 1% to 2% of sulfur, or its weightof a conversion product having no sulfur, to produce a product havingless than 3% of combined sulfur. If a soft rubber derivative is desired,the maximum of sulfur chemically combined in the rubber derivativecommensurate with requisite solubility may be about 2% or so, but thesame characteristics as to solubilizing prevail.

The characteristics of the derivatives produced are also somewhatdependent on-the rubberused as a starting material, andthe strongestadhesive is produced from rubber suchas-pale crepe, smoked sheet, etc.Masticated rubber may, however, be used as a basis for preparation ofthe derivative, but it is desirable that the amount of mastication bekept at a minimum value for the making of adhesives. Other rubberlikematerials, such as reclaim rubberand African or Congo rubber, maybe usedfor the production of rubber derivatives of the type herein disclosed.

The following examples, in whichthe parts are by weight, illustrate thepreparation of the rubber derivatives:

Example 1 Rubber 100 Acidified aluminum sulfate 100 Sulfur 3 Accelerator(tetramethylthiuramdisulfide) 2 These ingredients are thoroughly mixedon a rubber mill, sheeted to about 2.5 mm., heated at 150 C., for acouple of hours, and masticated to render the material soluble in rubbersolvents.

When phosphorus pentoxide is added to the above, the amount of acidaluminum sulfate may be decreased substantially, and improved materialis produced.

Example 2 A conversion product was prepared by intimately mixing 100parts of rubber, 12 parts of aluminum acid sulfate, 3 parts of P205, andl' art of water on a rubber mill. The material was sheeted out andheated as above. 100 parts of the conversion product thus produced weremilled with 1.5 parts of sulfur and .5 part of Captax(mercaptobenzothiazol) heated at vulcanizing temperature to produce thesulfur-containing rubber derivative, and masticated to render itsoluble. It may be dissolved in suitablerubber solvents and used as anadhesive, or it maybe mixed with suitable solvents, fillersand oils toproduce coatingsor paints.

If the derivative is to be used for the production of paints, it shouldpreferably be of the harder type and masticated a relatively, longertime to increase its solubility. The sulfur combines with the rubberconversion product and the resulting compound may be considered to havesubstantially the molecular formula RXA where R is a hydrocarbon derivedfrom rubber, A is one'of'the group consisting of sulfur, selenium, andtellurium, a: and :1; having numerical values.

During the first mastication after the exothermic reaction has takenplace, it is desirable to remove or neutralize any acid constituentsthat may be present. This may be accomplished by washing the materialand/or by adding a compound such as oxides or carbonates of magnesium,zinc, etc.

Any of the heretofore named conversion reagents may be substituted forthe aluminum acid sulfate and P205 in the above examples to producerubber conversion products. The derivatives produced with aluminum acidsulfate, alkali metal sulfates, or the alums, together with phosphoricacid, produce rubber conversion products having desirable properties forpreparation of adhesives which give relatively strong, heat resistantbonds between rubber and other surfaces.

The following examples illustrate the preparation of adhesives givingstrong bonds:

These ingredients were thoroughly mixed together on a'rubber mill,sheeted to about 2 or 3 mm. thickness or less, heated in vacuum for 2hours at 150 C. This material is relatively hard and was solubilized bymilling, mixed with 10 parts of zinc oxide, 30 parts carbon black, and2' parts of an antioxidant, and dissolved in an appropriate solvent toproduce an adhesive.

Example 4 Rubberlpale crepe) Acidified' aluminum sulfate 9 Phosphoruspentoxide 2.4 Water 1 Sulfur; 1.5

These ingredients were mixed as in Example 1, heated for 85 minutes atC. in vacuum. This material is softer than that produced in Example 3.It is compounded with 80 parts of zinc oxide and 2 parts of anantioxidant after which it is further milled for 30 minutes to render itcompletely soluble in rubber solvents.

In the preparation of assemblies wherein rubber is'vulcanized or bondedto metal or the like. a'strong'bond may be produced by the followingprocedure:

The metal is first coated with asolution of a harder rubber derivativesuch as that of Example 3, and then with a solution of a softerderivative suclras that of Example 4 after the previous coat has dried.One or more coats of each derivative may be applied but the harderderivative is put adjacent the metal and the softer derivative issuperimposed on the harder derivative. The rubber to be bonded to themetal may now be applied? and vulcanized directly thereto, a strong bondbein'g'produced.

It is preferable, however, to apply what is commonly termed a tie cementwhich may be formed by mixing 100 parts of rubber with suitablecompounding ingredients, such as about 40 parts of carbon black, 20parts short fibre asbestos, 1.25 parts stearic acid, 5 parts zinc oxide,with or with out vulcanizing agents and antioxidants, and dissolving thecompounded rubber in suitable solvents.

A suitable sample of metal was coated successively with the adhesives ofExamples 3, 4, and the tie cement above, each coat being allowed to drybefore the next was applied. The coated metal was placed in contact withvulcanizable rubber and the assembly heated in a suitable mold underpressure. This'assembly was removed from the hot mold. The strength ofthe bond at room temperature was 800 lbs. per square inch and 200 lbs.per square inch at 100 C. When the sulfur was omitted from the aboveexamples the conversion product and adhesive being otherwise identical,the strength of the bond produced as above was 775 lbs. per square inchat room temperature and about lbs. per square inch at 100 C.

The suitability of an adhesive for uniting rubber to metal and the likeis determined-to a large extent by the plasticity of the material atelevated temperatures. The plasticity of the material may be measured bysubjecting it to compression under a suitable stress such as onekilogram per square centimeter, measuring the deformation, thenreleasing the stress on the material and measuring its recovery. Ahigher ratio of recovery to deformation indicates the superior productwhich has less plasticity. The rubber derivatives produced according tothis invention and particularly those produced when an aluminum acidsulfate and P205 is used as the conversion reagent, have lessdeformation than rubber and are less plastic than rubber at elevatedtemperatures. Thus, a derivative produced with aluminum acid sulfate andP205 as in the above examples gave a recovery to deformation ratio of.80 at 120 0., whereas the ratio of recovery to deformation of palecrepe rubber is but .5 under similar conditions. Hydrocarbons derivedfrom rubber by the treatment with conversion reagents together withheating as above set forth have the same unit empirical formulaasrubber, namely Cal-Is, the number of these groups in one moleculebeing unknown, and the unit hydrocarbon R has less unsaturation thanrubber, as evidenced by the iodine values.

The rubber derivatives produced as above described are dissolved in asuitable solvent to produce adhesives suitable for bonding rubber tometal. These adhesives have improved tempera- ,ture resistance and agingqualities and provide for the formation of rubber to metal bonds havingincreased strength at elevated and ordinary temperatures. In theproduction of paints and the like the derivatives are dissolved insuitable solvents, and mixed with the desired pigments, oils, etc.

The rubber conversion product containing sulfur in combination andproduced according to the invention herein is particularly suitable forthe making of adhesives. Bonds made between rubber and metal and betweenrubber and other solids with adhesives made and applied according tothis invention have relatively high strength, are not thermoplastic, andare heat resistant. It has also been found that the rubber derivativesof this invention, when properly compounded with suitable pigments,etc., produce coating compositions and paints which adhere well and arevery stable.

The term unmelted rubber derivative as used herein includes only thosederivatives which have not been and are not melted.

It is obvious that various modifications may be made in the abovedescribed process without departing from the principles of the inventionherein set forth, and it is my intention not to limit the appendedclaims except as may be necessitated by the prior art.

What I claim is:

1. A relatively uniform, unmelted, soluble, solid, masticatedhomogeneous heat reaction product formed by heating in vacuum arelatively thin sheet of a mixture of unvulcanized rubber, a salt of astrong acid, a weak acid, and less than 3% sulphur.

2. A method for producing soluble, unmelted, solid rubber derivativeswhich are less plastic than rubber at temperatures around 100 C. andwhich contain a vulcanizing agent combined therewith, which comprisesforming an intimate mixture of a member of the group consisting ofsulfur, selenium and tellurium with a solid, relatively non-volatilesalt of a strong acid, rubber,

not substantially greater than 5 mm., heatingthe formed mix to atemperature between about C. and about C. to procure an exothermicreaction product, and masticating the product to confer solubility insolvents for rubber.

3. A method for producing soluble, unmelted, solid rubber derivatives,containing a vulcanizing agent combined therewith, which are lessplastic than rubber at elevated temperatures, which comprises forming anintimate mixture of sulfur, a solid inorganic salt of a strong acid,crude rubber, water, including water of crystallization, and a weaklyacidic substance, forming the mixture into thin sections having at leastone dimension not substantially greater than 5 mm, and heating theformed mix to a temperature of between about 100 C. and 170 C. toprocure an exothermic reaction product, said sulfur not exceedingsubstantially 3% of the weight of said rubber.

4. A method for producing soluble, solid, unmelted rubber derivativescontaining a vulcanizing agent combined therewith, which comprisesforming an intimate mixture of selenium with a solid, non-volatile,inorganic metal salt of a strong acid, unvulcanized rubber, water,including water of crystallization, and a weakly acidic substance,forming the mixture into thin sections having at least one dimension notsubstantially greater than 5 mm., heating the formed mix to atemperature of between about 100 C. and about 170 C. to procure anexothermic reaction product, and masticating the product sufficiently toconfer solubility.

5. A solid, substantially homogeneous, unmelted derivative of solidrubber which is soluble in rubber solvents derived from petroleum andwhich is characterized in dissolved form by being suitable for adheringrubber to metal, said derivative having the same carbon to hydrogenratio as rubber and containing in combined form a vulcanizing elementfor rubber selected from the group consisting of sulfur, selenium andtellurium, said rubber derivative being further characterized by havingless plasticity than pale crepe rubber at temperatures of the order of100 C. and by being a masticated heat reaction product of a mixture ofsolid rubber, a salt of a strong acid, a vulcanizing element and aweakly acidic substance, the amount of said element chemically combinedin said derivative being not substantially greater than 3%.

6. A relatively uniform, unmelted, solid, masticated, homogeneous heatreaction product formed by heating in an atmosphere having less oxygenpressure than does air a relatively thin sheet of a mixture containingsolid rubber, a salt of a strong acid, a Weak acid and less than about3% of sulfur, said masticated heat reaction product being soluble inpetroleum solvents for rubber.

7. A solid, unmelted, masticated derivative of solid rubber containingcombined sulfur and having the same carbon to hydrogen ratio as rubber,said derivative being obtained by heat ing in a form having thinsection, a previously undissolved mixture of solid rubber, water,including Water of crystallization, a metal salt of a strong acid, aweak acid and sulfur to procure a reaction, and masticating the productto solubilize it in petroleum solvents for rubber, the amount of sulfurchemically combined with said derivative being less than 3%.

8. A method of producing soluble, unmelted, solid, rubber derivatives,which comprises intimately mixing sulfur, a metal sulfate and water,including water of crystallization, with solid rubber, forming themixture into relatively thin section having at least one dimension notgreater than about 5 mm, heating the mixture to about 100 C. to initiatea reaction, and masticating the product to render it soluble inpetroleum solvents for rubber, the amount of said sulfur chemicallycombined With said derivatives being not more than 3%.

9. A substantially homogeneous, unmelted, solid, cyclized rubberderivative obtained by heating rubber with a catalyst comprising a saltof a strong acid and Water, and characterized by having less plasticitythan pale crepe at temperatures of about 100 C., by containing sulfur inan amount up to 3% and, after mastication, by being soluble in petroleumsolvents for rubber.

10. A homogeneous, unmelted, solid derivative of solid rubber which,after mastication, is soluble in rubber solvents and which is'the heatreaction product in a section having a thickness not substantiallygreater than 5 mm. of a mixture comprising essentially solid rubber, arelatively non-volatile acid salt of a strong acid and water, and up to3% of sulfur.

11. A homogeneous, unmelted solid derivative of solid rubber which,after mastication, is soluble in petroleum solvents for rubber, andwhich is the heat reaction product in a section having a thickness notsubstantially greater than 5 mm. of a mixture comprising essentiallysolid rubber, aluminum sulfate, phosphorus pentoxide and up to 3% of amember of the group consisting of sulfur, selenium and tellurium.

12. A homogeneous, unmelted, solid derivative of solid rubber which,after mastication, is soluble in petroleum solvents for rubber, andwhich is the heat reaction product in a section having a thickness notsubstantially greater than 5 mm. of a mixture comprising essentiallysolid rubber, acid aluminum sulfate, water, and less than 3% of sulfur.

THOMAS RAYMOND GRIFFITH.

